A membrane of the prior art is seen in Neu Pat. No. 4,608,164 and is reflected in the Hansen membrane filter plate sold by Hansen-BTR of West Germany. This patent includes an elastomeric (rubber) membrane anchored in a transition zone between a peripheral seal surface of the filter plate and a central recessed filter plate portion. The outer peripheral surface of the membrane adjacent the plate seal surface is higher than the plate seal surface so that when adjacent plates are brought together in a multiplate or plate and frame filter press, the membrane periphery is compressed providing a pressure seal between adjacent plates. In the '164 patent a groove is provided around the entire plate including the outlet and inlet and the membrane is formed to generally coincide with the entire operating portion of the plate, i.e. the plate and membrane are designed together so that for any change in plate porting a new membrane must be designed to correspond to the plate porting. The '164 patent and particularly the Hansen membrane plate is designed such that at the outer periphery and particularly around the center porting there is a tendency of adjacent membrane portions to suffer considerable elongation (of the order of 30-40%) and thus shorten the lifespan and decrease the efficiency of the membrane. Neu recognizes that membrane folding shortens the useful life (col. 1 lines 61-63). This characteristic is particularly seen in FIG. 4 hereof which illustrates the prior art in which high elongation levels of membranes 30, 31 are present at points A, B and C when the membranes are in the squeeze cycle particularly with little filter cake therebetween. Portions 32 of the membrane are forced by air pressure effecting the squeezing of the cake, into the feed passages leading to the feed eye 34 despite the use of members 33 having curved radiused (r.sub.2) outer edges as see in the lower half of FIG. 5 in the '164 patent. These do not provide effective support of the membranes to minimize elongation.
Further, as seen in FIGS. 13, 14 and 17 of the '164 patent a complex series of ports and grooves must be molded in the respective inlet and outlet corners of the thus perforated membrane, which results in added costs, material wastage, a greater potential for leaks and a loss of flexibility when using plates with different porting arrangements.